The synthesis, characterisation and reactivity of 2-phosphanylethylcyclopentadienyl complexes of cobalt, rhodium and iridium

Abstract

2-Phosphanylethylcyclopentadienyl lithium compounds, Li[C₅R′₄(CH₂)₂PR₂] (R = Et, R′ = H or Me, R = Ph, R′ = Me), have been prepared from the reaction of spirohydrocarbons C₅R′₄(C₂H₄) with LiPR₂. C₅Et₄HSiMe₂CH₂PMe₂, was prepared from reaction of Li[C₅Et₄] with Me₂SiCl₂ followed by Me₂PCH₂Li. The lithium salts were reacted with [RhCl(CO)₂]₂, [IrCl(CO)₃] or [Co₂(CO)₈] to give [M(C₅R′₄(CH₂)₂PR₂)(CO)] (M = Rh, R = Et, R′ = H or Me, R = Ph, R′ = Me; M = Ir or Co, R = Et, R′ = Me), which have been fully characterised, in many cases crystallographically as monomers with coordination of the phosphorus atom and the cyclopentadienyl ring. The values of ν_CO for these complexes are usually lower than those for the analogous complexes without the bridge between the cyclopentadienyl ring and the phosphine, the exception being [Rh(Cp′(CH₂)₂PEt₂)(CO)] (Cp′ = C₅Me₄), the most electron rich of the complexes. [Rh(C₅Et₄SiMe₂CH₂PMe₂)(CO)] may be a dimer. [Co₂(CO)₈] reacts with C₅H₅(CH₂)₂PEt₂ or C₅Et₄HSiMe₂CH₂PMe₂ (L) to give binuclear complexes of the form [Co₂(CO)₆L₂] with almost linear PCoCoP skeletons. [Rh(Cp′(CH₂)₂PEt₂)(CO)] and [Rh(Cp′(CH₂)₂PPh₂)(CO)] are active for methanol carbonylation at 150 °C and 27 bar CO, with the rate using [Rh(Cp′(CH₂)₂PPh₂)(CO)] (0.81 mol dm⁻³ h⁻¹) being higher than that for [RhI₂(CO)₂]⁻ (0.64 mol dm⁻³ h⁻¹). The most electron rich complex, [Rh(Cp′(CH₂)₂PEt₂)(CO)] (0.38 mol dm⁻³ h⁻¹) gave a comparable rate to [Cp*Rh(PEt₃)(CO)] (0.30 mol dm⁻³ h⁻¹), which was unstable towards oxidation of the phosphine. [Rh(Cp′(CH₂)₂PEt₂)I₂], which is inactive for methanol carbonylation, was isolated after the methanol carbonylation reaction using [Rh(Cp′(CH₂)₂PEt₂)(CO)]. Neither of [M(Cp′(CH₂)₂PEt₂)(CO)] (M = Co or Ir) was active for methanol carbonylation under these conditions, nor under many other conditions investigated, except that [Ir(Cp′(CH₂)₂PEt₂)(CO)] showed some activity at higher temperature (190 °C), probably as a result of degradation to [IrI₂(CO)₂]⁻. [M(Cp′(CH₂)₂PEt₂)(CO)] react with MeI to give [M(Cp′(CH₂)₂PEt₂)(C(O)Me)I] (M = Co or Rh) or [Ir(Cp′(CH₂)₂PEt₂)Me(CO)]I. The rates of oxidative addition of MeI to [Rh(C₅H₄(CH₂)₂PEt₂)(CO)] and [Rh(Cp′(CH₂)₂PPh₂)(CO)] are 62 and 1770 times faster than to [Cp*Rh(CO)₂]. Methyl migration is slower, however. High pressure NMR studies show that [Co(Cp′(CH₂)₂PEt₂)(CO)] and [Cp*Rh(PEt₃)(CO)] are unstable towards phosphine oxidation and/or quaternisation under methanol carbonylation conditions, but that [Rh(Cp′(CH₂)₂PEt₂)(CO)] does not exhibit phosphine degradation, eventually producing inactive [Rh(Cp′(CH₂)₂PEt₂)I₂] at least under conditions of poor gas mixing. The observation of [Rh(Cp′(CH₂)₂PEt₂)(C(O)Me)I] under methanol carbonylation conditions suggests that the rhodium centre has become so electron rich that reductive elimination of ethanoyl iodide has become rate determining for methanol carbonylation. In addition to the high electron density at rhodium.